Generally, semiconductor lasers having Fabry-Perot (FP) resonator configuration are used, the resonator being formed by mirrors on the both end faces of an active layer. However, the FP lasers produce laser light at a wavelength that satisfies conditions for forming standing waves. Thus, the lasers tend to operate in a multi-longitudinal mode. Particularly as the current and the temperature changes, the laser oscillation wavelength varies, which results in a change in optical intensity.
Thus, in applications in optical communication, gas sensing, and the like, there has been a need for a laser that exhibits high wavelength stability and that operates in a single mode. Thus, distributed feed-back (DFB) lasers and distributed reflection (DBR) lasers have been developed. These lasers include a grating in a semiconductor element and produce only laser light having a specific wavelength by using the wavelength dependency of the grating.
Examples of semiconductor lasers that exhibit wavelength stability can include DBR lasers and DFB lasers having a grating that is monolithically formed in the semiconductor laser and external resonator lasers having a fiber grating (FBG) grating disposed on the exterior of the laser. The principle of these lasers is that a part of laser light is returned to the lasers by wavelength-selective mirrors that use Bragg reflection to achieve wavelength stable operation.
DBR lasers include ridges and grooves that are formed in a surface of a waveguide lying on the same straight line as a waveguide in the active layer and mirrors that uses Bragg reflection for realizing a resonator (Patent Document 1 (Japanese Unexamined Patent Application Publication No. S49-128689A) and Patent Document 2 (Japanese Unexamined Patent Application Publication No. S56-148880A)). These lasers include a grating on the both end surfaces of the optical waveguide layer. Thus, light emitted by the active layer is propagated through the optical waveguide layer, and a part of the light is reflected by the grating and is returned to the current injection portion, where the light is amplified. Only light having a single wavelength is reflected by the grating in a predetermined direction, and thus laser light has a constant wavelength.
As an application of such lasers, external resonator semiconductor lasers that include the grating as a separate component from a semiconductor element to form an external resonator have been developed. This type of lasers exhibit good wavelength stability, temperature stability, and controllability. Examples of the external resonator include fiber Bragg gratings (FBG) (Non-Patent Document 1) and volume holographic gratings (VHG) (Non-Patent Document 2). The gratings are configured to be a separate component from the semiconductor lasers, and thus the lasers are characterized in that the reflectance and the resonator length can be independently designed. As the gratings are not affected by temperature rise due to heat generation caused by current injection, the wavelength stability can be further improved. As the semiconductor material has a different temperature dependency of the refractive index, the temperature stability can be improved by designing the refractive index together with the length of the resonator.
Patent Document 6 (Japanese Unexamined Patent Application Publication No. 2002-134833A) discloses an external resonator laser that uses a grating formed in a silica glass waveguide. The patent is to provide a frequency-stable laser that can be used, without a temperature controller, in an environment in which room temperature significantly changes (for example 30° C. or more). The patent describes provision of a temperature-independent laser that prevents mode hopping and that does not depend on temperature for laser oscillation frequency.
Patent Document 8 (Japanese Unexamined Patent Application Publication No. 2010-171252A) discloses an external resonator laser that includes an optical waveguide having a core layer of SiO2, SiO1-xNx (wherein x is from 0.55 to 0.65), or Si and SiN and a grating formed on the optical waveguide. The external resonator laser maintains a constant laser oscillation wavelength without precise temperature-control and presupposes that a change in reflection wavelength with temperature (temperature coefficient of the Bragg reflection wavelength) of the grating is decreased. Based on this, the patent describes that operation of the laser in a multi-longitudinal mode can provide power stability.
Patent Document 9 (Japanese Patent No. 3667209B) discloses an external resonator laser that uses a grating formed in an optical waveguide of quartz, InP, GaAs, LiNbO3, LiTaO3, or polyimide resin. The patent described that the reflectance of the light-emitting surface of the semiconductor laser, which is a light source, is the effective reflectance Re (substantially from 0.1 to 38.4%), and that based on this, operation of the laser in a multi and longitudinal mode can provide power stability.